44 
The American Geologist. 
Jaiinary, 1905 
darken the field. In a few instances it was necessary to 
refer to the section to establish the identity of certain grains 
in the prints. The various minerals in each of the six 
prints were trimmed apart and weighed. These weights, 
following the methods of Delesse and Sollas, were assumed 
proportional to the areas and hence to the volumes of the 
■minerals, and the percentage composition computed as be- 
fore. The average composition of the six fields by weight 
is included in the table. 
(3) MICROSCOPIC MEASUREMENT OF AREAS OF GRAINS. 
The third process mentioned consists in directly measur- 
ing the area of each grain of the minerals of a section with 
the microscope. This is done by the application of an eye- 
piece micrometer which is cross ruled into numerous small 
squares. The number of these squares coming within the 
boundaries of each grain in a field is counted. The areal 
proportions of the cross sections of the minerals are thus 
obtained. The fields for measurement are to be contiguous 
and taken in regular order in a given direction across the sec- 
tion ; the slide being moved by means of a mechanical stage 
successively the width of a field as each is measured. 
The data given below were obtained with fields of 4 
millimetres actual diameter, and a magnification of 30. The 
basis of the calculations is here again the assumption that 
the average cross section area is proportional to volumes. 
Mineral 
Components 
Quartz 
Orthoclase 
Albite 
Anorthite . . . , 
Mica 
Magnetite . . . . 
Hematite 
Mineral Composition by Weight Determined by 
Calf, 
from 
Chem. 
Anal. 
35.90 
34.48 
18 74 
6.25 
2.55 
2.605 
1,10 
Heavy 
Solu- 
tion 
34.25 
32 465 
( 26.905 
5.68 
Meas 
of Dl 
ameter 
d 
Calf, of 
d3 
from 
last 
19.025 
43.645 
83.897 
4.43 
1.406 
56.04 
42.24 
0.31 
Meas. 
Photo- I of 
graphyl Areas 
d2 
23,23 
43.64 
32.90 
0.25 
19 901 
43.67 
32.896 
3.526 
Calc of 
d3 
from 
last 
12.14 
40,92 
37.81 
0.68 
In this array of results for the mineral analysis of a sin- 
gle rock, the most striking comparison is, perhaps, between 
the heavy solution and chemical analysis as one group, and 
the microscopic methods as a group by themselves. The re- 
